Such an asymmetrical grid fault is present when at least the amplitude of an electrical variable in one phase of the grid deviates from the nominal value. The cause of an asymmetrical grid fault may be, for example, a short circuit in the corresponding phase. In accordance with the method of symmetrical components, an asymmetrical grid fault is defined by the occurrence of components differing from zero in the negative phase-sequence system. In the method of symmetrical components, a three-phase system is transformed into a two-phase system, wherein a first phasor corresponds to the positive phase-sequence system and a second phasor corresponds to the negative phase-sequence system. The phasors contain the active and reactive power components of the respective system in their real and imaginary parts. The components thus constitute a measure for the asymmetry of a grid fault and enable classification of such faults and therefore fault clearance.
E VDE-AR-N 4120, Draft, November 2012 entitled “Technische Bedingungen für den Anschluss and Betrieb von Kundenanlagen an das Hochspannungsnetz (TAB Hochspannung)” {Technical requirements for the connection and operation of customer installations to the high voltage networks (TAB high voltage) http://www.vde.com/en/dke/std/VDEapplicationguides/Publications/Pages/VDE-AR-N4120.aspx} discloses a voltage support in the case of grid faults via a reactive current feed-in. Chapter 10.2.3.2 explains that, in the event of the occurrence of sudden voltage deviations of ΔU≧±2.5% Unominal, generating units are required to support the voltage of the high-voltage grid by matching (increasing or decreasing) the reactive current IB fed into the grid by providing an additional reactive current ΔIB. For ΔU<±2.5% Unominal, the requirements for steady-state voltage stability apply. More detailed explanations are provided with respect to the fact that the distribution of the reactive current deviation ΔIB to a positive phase-sequence system and a negative phase-sequence system (ΔiB1, and ΔiB2) would have to take place corresponding to the change in voltage in the positive phase-sequence system and in the negative phase-sequence system, respectively.
U.S. Pat. No. 8,390,138 discloses a wind turbine whose line-side converter comprises a negative phase-sequence system control system including a phase control module. The phase control module is configured to determine an electrical variable of the negative phase-sequence system in a phase-specific manner. Thus, the available current can be preset depending on the operating condition for active power or reactive power in the negative phase-sequence system. Phase control can be used for stabilization of the grid in particular in the case of asymmetrical grid conditions.
U.S. Pat. Nos. 8,390,139 and 8,400,003 disclose a method for controlling a converter of a wind turbine having a doubly-fed induction generator. The wind turbine comprises a generator-side and a line-side converter. A change in the reactive current output of one converter is detected and, in the event of a deviation from a setpoint value, the setpoint value for the reactive current to be output by the respective other converter is changed. The control of the two converters is matched to one another in such a manner that the total current output is optimized.
Lie, Xu: “Coordinated Control of DFIG's Rotor and Grid Side Converters During Network Unbalance”, IEEE Transactions on Power Electronics, vol. 23, no. 3, May 2008 discloses a method for controlling an electrical generating system having a doubly-fed induction generator in the case of the occurrence of an asymmetrical line voltage. The rotor-side converter is controlled in such a manner that torque variations are suppressed. Fluctuations in the stator active power output into the grid are compensated by control of the active power output of the line-side converter, resulting in the generating system feeding a constant active power into the grid. The control method is based on a mutually dependent control of the currents in the positive phase-sequence system and the negative phase-sequence system of the rotor-side and the line-side converter.